1. Field of the Invention
This invention relates generally to differential amplifiers, and more particularly to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit to achieve high performance as compared to amplifiers with class AB differential input stages that employ two loop solutions which have loop interplay.
2. Description of the Prior Art
Differential amplifiers are very useful devices when dealing with differential systems. They have several advantages such as very good common mode rejection ratio since everything that is common will get canceled at the differential output. Some applications of differential amplifiers include single ended to differential conversion, differential ADC driver, differential transmitter and receiver, and output level shifter.
Differential amplifiers are not too different from single ended amplifiers. Several single ended topologies can be converted to differential topologies. The most obvious difference between differential amplifiers and their single ended counterparts is that they have two outputs instead of one. With the addition of this output, another difference arises, which is the xe2x80x9coutput common mode feedback circuitxe2x80x9d (CMFC). Extra circuitry is necessary to direct the outputs where to go DC wise. Since this signal is common to both outputs differentially, it will cancel, leaving at the differential output only the amplified differential input signal.
Implementing such a common mode circuit is challenging. FIG. 1 illustrates one implementation of a differential amplifier 100 that includes an emitter coupled pair input stage 102 with a folded cascode gain stage 104. The CMFC includes an error amplifier 106 that compares the common mode voltage at the outputs to the voltage set on the Vocm pin 108 (seen defaulted to mid rail in FIG. 1). The error amplifier 106 adjusts the common mode voltage by changing the voltage in the bases of transistors Q5 and Q6. The feedback path of the error amplifier 106 in FIG. 1 is from the output of the error amplifier 106 to the bases of transistors Q5 and Q6, to the outputs of the main amplifier through the buffers 110, 112, and back into the positive input of the error amplifier 106.
The class AB input stage in a differential configuration has several advantages over the class A single emitter coupled pair input stage illustrated in FIG. 1. Some of these advantages include better slew rate and better THD performance, especially at high frequencies. It would be desirable to have a way of implementing the foregoing CMFC on this topology. FIG. 2 is a schematic diagram illustrating a differential class AB amplifier 200 without the CMFC. It can be seen this topology is quite different from the one shown in FIG. 1. One of the primary differences between this kind of circuit and the circuit in FIG. 2 is that in FIG. 1 it is only necessary to manipulate the current of transistors Q5 and Q6 to control the output common mode voltage. In the circuit of FIG. 2, it is necessary to manipulate the top mirrors Q5, Q6 and Q13, Q14 as well as the bottom mirrors Q7, Q8 and Q15, Q16. The conventional prior art solution includes the use of two loops to control the two sets of mirrors, and that way control the output common mode voltage. This conventional approach has been problematic however since it results in undesirable distortion associated with additional parasitics and mismatch between the loops, stability problems due to complexity of the multiple control loops and the interaction between them, added complexity, and circuitry that consumes additional die area.
In view of the foregoing, it would be desirable to have a way of implementing the CMFC on a class AB input stage in a differential configuration in a manner that does not result in additional distortion, or stability problems due to increased complexity of multiple control loops, and that does not undesirably increase the required die area.
The present invention is directed to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit (CMFC) to achieve high performance by controlling the common mode output voltage. The CMFC includes a small amplifier to compare the voltage at the outputs with a desired voltage. Having only one loop to control this voltage instead of two makes the design more reliable and easier to compensate since there is no need to worry about how multiple loops will interact. The distortion caused by the nonlinear parasitics associated with the CMFC are minimized by connecting the CMFC to a point at the input where the voltage does not change as much as in the output of the input stage, where the full voltage signal is seen.
According to one aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry having only one loop to control the output common mode voltage;
According to another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that distortion caused by the CMFC is minimized;
According to yet another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that interaction between multiple loops caused by a CMFC is eliminated;
According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the CMFC is less complex and requires less die area than known two loop CMFC implementations associated with class AB differential amplifiers.
According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the resultant class AB differential amplifier power consumption is reduced when compared with conventional class AB differential amplifier architectures that employ two loops to control the common mode output voltage.